Tearing of N115-Filled Natural Rubber Vulcanizates at Various Temperatures

Natural rubber vulcanizates have high tear resistance, excellent flexibility and resilience. All tires contain black-filled natural rubber compounds in the location (belt region) of maximum stress concentration.

Specimens of gum natural rubber vulcanizates show an abrupt drop in strength at a critical cut size and an abrupt drop of normal (no cut) tensile strength at a critical temperature. These effects are due to bulk strain crystallization at small cut sizes or low temperatures¹. The addition of carbon black affects the tearing behavior and depends on the type and concentration of black. Natural rubber vulcanizates filled with high concentrations carbon black have very high cut resistance. This is due to a high degree of crystallization around the cut tip^{2, 3}.  

The tearing behavior of a series of natural rubber vulcanizates containing a very fine carbon black (N115) were tested at various temperatures. Amazingly, the addition of low concentrations of carbon black decreases the cut resistance. There is a critical level of black needed for reinforcement. The tearing behavior in the transition zone depends on temperature. At sufficient high temperatures, black-filled natural rubber vulcanizates behave like amorphous polymers; they show a steady increase in strength as carbon black content is increasing. Crack patterns also were investigated. Black-filled natural rubber vulcanizates show increased complexity of cracking with increasing temperature.

References

1. A. G. Thomas and J. M. Whittle, Rubber Chem Technol., 43, 222 (1970)

2. G. R. Hamed and B. H. Park, Rubber Chem Technol., 72, 946 (1999)

3. D. J. Lee and J. A. Donovan, Rubber Chem Technol., 60, 910 (1987)